Thunderbird can support dozens of accounts, each with their own SMTP server. It is not clear if there is any limit to the number of accounts, I know of one case with 150 accounts.

When you add an account using the new account wizard it configures the account to use the email providers SMTP server. But you can edit the accounts server settings afterwards to use any of the known SMTP servers. There is a "default" SMTP server setting but that setting is ignored if you use the new account wizard, its there just for the small percentage of users who manually add a account (without the wizard) using an add-on such as "add mail account manually" .

You can configure multiple POP accounts to use a "global inbox", which hides the accounts folders in the folder pane, and uses the "Local Folders" accounts folders instead. But even then you can still have separate SMTP servers for each of those accounts.